Highly corrosion-resistant hot-dip coated steel product excellent in surface smoothness and formability and method for producing hot dip coated steel product

ABSTRACT

The present invention provides a highly corrosion-resistant plated steel sheet that can achieve excellent surface smoothness and formability and, according to the process of the present invention, a hot-dip galvanized steel product excellent in surface smoothness and formability having on the steel product surface a zinc alloy plating layer composed of 4 to 22% by mass of Al, 1 to 5% by mass of Mg, 0.000001 to 0.1% by mass of Ti, 0.000001 to 0.5% by mass of Si and the balance of Zn and unavoidable impurities, the plating layer of the plated steel product having a metal structure in which an [Mg 2 Si phase], an [Al phase], a [Zn 2 Mg phase] and a [Zn phase] are present in a mixture in the matrix of an [Al/Zn/Zn 2 Mg ternary eutectic structure], and the plating layer containing a Ti—Al base intermetallic compound in the [Al phase] and/or the [Zn 2 Mg phase] and/or the [Zn phase], is produced.

FIELD OF THE INVENTION

The present invention relates to a plated steel sheet and, in moredetail, to a highly corrosion-resistant plated steel product that can beapplied to various applications and, for example, to householdelectrical appliances, automobiles and steel sheets for buildingmaterials.

BACKGROUND ART

There are zinc base plated steel sheets that are often used as platedsteel products excellent in corrosion resistance. The plated steelsheets are used in various manufacturing industries such as theautomobile, household electrical appliance and building materialindustries. Moreover, plated steel products are used in various otherfields such as the plated steel wire and hot-dip galvanized steelproduct fields.

In order to improve the corrosion resistance of such zinc base platedsteel products, the present inventors have proposed a hot-dipZn—Al—Mg—Si coated steel sheet (see, e.g., Japanese Patent PublicationNo. 3,179,446).

Furthermore, in order to improve the corrosion resistance of zinc baseplated steel products, a zinc base plated steel sheet that is madeexcellent in age-based darkening resistance by adding Ti to the hot-dipZn—Al coated steel sheet has been proposed (see, e.g., JapaneseUnexamined Patent Publication (Kokai) No. 5-125515). However, theproblem that the resultant plated steel sheet shows poor surfacesmoothness and formability was not taken into consideration.

Still furthermore, a zinc-base plated-steel sheet, the appearance ofwhich is made good by adding Ti, B and Si to a hot-dip Zn—Al—Mg coatedsteel sheet, has been proposed (e.g., see Japanese Unexamined PatentPublication (Kokai) No. 2001-295015). Although Ti and B are added forthe purpose of inhibiting the formation and growth of a Zn₁₁Mg₂ phasethat makes the appearance of the plated steel sheet poor, the proposalneither considers the problem that the addition makes the surfacesmoothness and formability of the plated steel sheet poor, nor refers toformation of an intermetallic compound.

However, the surface smoothness and formability are not ensuredsufficiently for the above plated steel sheets and other disclosedplated sheets.

When the solidification rate of the plating layer is adequately ensuredduring hot-dip galvanizing, the plating layer solidifies before the Alphase grows significantly. As a result, a problem of the surfacesmoothness does not arise. However, when the solidification rate of theplating layer is small, the Al phases significantly grow first.Protrusions and recesses are then formed on the plating layer surface,and the resultant plated steel sheet has the problem that the surfacesmoothness and formability become poor.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve the above problems, andprovide highly corrosion-resistant plated steel products.

As a result of intensively carrying out investigations on thedevelopment of a plated steel sheet excellent in surface smoothness andformability, the present inventors have discovered that the surfacesmoothness and formability can be improved by making the plating layerhave a metal structure in which one or at least two of the [Mg₂Siphase], [Al phase], [Zn₂Mg phase] and [Zn phase] are present in amixture in the matrix of an [Al/Zn/Zn₂Mg ternary eutectic structure],and making one or at least two of the [Al phase], [Zn₂Mg phase] and [Znphase] contain a Ti—Al base intermetallic compound, and they have thusachieved the present invention. The aspects of the present invention areas described below.

(1) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, having on the steelproduct surface a zinc alloy plating layer composed of 4 to 10% by massof Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti and the balanceof Zn and unavoidable impurities, the plating layer having a metalstructure in which one or at least two of the [Al phase], [Zn₂Mg phase]and [Zn phase] are present in a mixture in the matrix of an [Al/Zn/Zn₂Mgternary eutectic structure], and the plating layer containing a Ti—Albase intermetallic compound in one or at least two of the [Al phase],[Zn₂Mg phase] and [Zn phase].

(2) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, having on the steelproduct surface a zinc alloy plating layer composed of 4 to 22% by massof Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti, up to 0.5% bymass of Si and the balance of Zn and unavoidable impurities, the platinglayer of the plated steel product having a metal structure in which an[Mg₂Si phase], an [Al phase] and a [Zn₂Mg phase] are present in amixture in the matrix of an [Al/Zn/Zn₂Mg ternary eutectic structure],and the plating layer containing a Ti—Al base intermetallic compound inone or at least two of the [Al phase] and the [Zn₂Mg phase].

(3) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, having on the steelproduct surface a zinc alloy plating layer composed of 4 to 22% by massof Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti, up to 0.5% bymass of Si and the balance of Zn and unavoidable impurities, the platinglayer of the plated steel product having a metal structure in which an[Mg₂Si phase], an [Al phase], a [Zn₂Mg phase] and a [Zn phase] arepresent in a mixture in the matrix of an [Al/Zn/Zn₂Mg ternary eutecticstructure], and the plating layer containing a Ti—Al base intermetalliccompound in one or at least two of the [Al phase], [Zn₂Mg phase] and [Znphase].

(4) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability having, on the steelproduct surface, a zinc alloy plating layer composed of 4 to 22% by massof Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti, up to 0.5% bymass of Si and the balance of Zn and unavoidable impurities, the platinglayer of the plated steel product having a metal structure in which an[Mg₂Si phase], an [Al phase] and a [Zn phase] are present in a mixturein the matrix of an [Al/Zn/Zn₂Mg ternary eutectic structure], and theplating layer containing a Ti—Al base intermetallic compound in one ortwo of the [Al phase] and [Zn phase].

(5) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, wherein the Ti—Al baseintermetallic compound according to any one of (1) to (4) mentionedabove is TiAl₃.

(6) A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, wherein the Ti—Al baseintermetallic compound according to any one of (1) to (4) mentionedabove is Ti(Al_(1-x)Si_(x))₃ (wherein X=0 to 0.5).

(7) The highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability according to any one of(1) to (6) mentioned above, wherein the Ti—Al base intermetalliccompound contained in an [Al phase] in the plating layer is present in aZn—Al eutectoid reaction structure in which Zn phases are condensed.

(8) The highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability according to any one of(1) to (7) mentioned above, wherein the size of a dendrite in an [Alphase] in the plating layer is up to 500 μm.

(9) A process for producing the highly corrosion-resistant hot-dipgalvanized steel product excellent in surface smoothness and formabilityaccording to any one of (1) to (8) mentioned above, comprising the stepof adding a Ti—Zn base intermetallic compound to a plating bath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a photomicrograph substituted for drawing (magnification:1,000×) of a plating layer of the plated steel product of the presentinvention, and

FIG. 1(b) is a view showing the distribution state of each structure inthe photomicrograph.

FIG. 2(a) is a photomicrograph substituted for drawing (magnification:3,500×) that enlarges the [Al″ phase] in FIG. 1, and FIG. 2(b) is a viewshowing the distribution state of each structure in the photomicrograph.

BEST MODE FOR CARRYING OUT THE INVENTION

The hot-dip galvanized steel product according to the present inventionis a plated steel sheet having either a plating layer composed of 4 to10% by mass of Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti andthe balance of Zn and unavoidable impurities, or a plating layercomposed of 4 to 22% by mass of Al, 1 to 5% by mass of Mg, up to 0.1% bymass of Ti, up to 0.5% by mass of Si and the balance of Zn andunavoidable impurities, the plating layer of the plated steel sheethaving a metal structure in which one or at least two of the [Mg₂Siphase], [Al phase], [Zn₂Mg phase] and [Zn phase] are present in amixture in the matrix of an [Al/Zn/Zn₂Mg ternary eutectic structure],and the plating layer containing a Ti—Al base intermetallic compound inone or at least two of the [Al phase], [Zn₂Mg phase] and [Zn phase].

The Al content in the Zn—Al—Mg—Ti base plating layer is restricted to 4to 10% by mass for the following reasons. When the Al content exceeds10% by mass, the adhesion of the plating layer decreases. The Al contentin the plating layer to which Si is not added must therefore be made 10%by mass or less. Moreover, when the Al content is less than 4% by mass,no Al phase crystallizes as primary crystals, and the problem oflowering of smoothness does not arise.

Accordingly, in the hot-dip galvanized steel product of the invention,it is essential to add Si to the plating layer to ensure adhesion of theplating layer particularly when the Al concentration is as high asgreater than 10% by mass.

On the other hand, the Al content in the Zn—Al—Mg—Ti—Si base platinglayer is restricted to 4 to 22% by mass for the following reasons. Whenthe Al content is less than 4% by mass, no problem about lowering ofsmoothness arises because no Al phase crystallizes as primary crystals.When the Al content exceeds 22% by mass, the effect of improving thecorrosion resistance is saturated.

The Si content is restricted to 0.5% by mass or less (Si content of 0%by mass being excluded) for the following reasons: although Si has theeffect of improving the adhesion, the effect is saturated when thecontent exceeds 0.5% by mass. The Si content is desirably from 0.00001to 0.5% by mass, and more desirably from 0.0001 to 0.5% by mass.

Addition of Si to a plating layer having an Al content exceeding 10% bymass is essential. However, even for a plating layer having an Alcontent of up to 10%, because the effect of improving the adhesion ofthe plating layer is also significant, addition of Si to the platinglayer of a steel product is effective when the steel product is requiredto have high adhesion of the plating layer, for example, when the steelproduct is used as a member to be severely worked. Moreover, as a resultof adding Si, a [Mg₂Si phase] crystallizes in the solidificationstructure of the plating layer. Because the [Mg₂Si phase] has the effectof improving the corrosion resistance of a worked portion, it is moredesirable to increase an addition amount of Si so that a metal structurein which the [Mg₂Si phase] is present in a mixture in the solidificationstructure of the plating layer is formed.

The Mg content is restricted to 1 to 5% by mass for the followingreasons: when the Mg content is less than 1% by mass, the effect ofimproving the corrosion resistance is inadequate; when it exceeds 5% bymass, the plating layer is embrittled, and the adhesion thereofdecreases.

The Ti content is restricted to 0.1% by mass or less (Ti content of 0%by mass being excluded) for the following reasons. Ti has the effect ofcrystallizing a Ti—Al base intermetallic compound and improving thesurface smoothness and formability. However, when the Ti content exceeds0.1% by mass, the steel product after plating has a rough surface, andit has a poor appearance. Moreover, when the Ti content exceeds 0.1% bymass, a Ti—Al base intermetallic compound is condensed in the platinglayer surface to decrease the surface smoothness and formability. The Ticontent is desirably from 0.00001 to 0.1% by mass. The Ti content ismore desirably from at least 0.00001% by mass to less than 0.01% bymass.

For the plated steel product according to the present invention, a metalstructure containing at least one of the [Zn phase], [Al phase], [Zn₂Mgphase], [Mg₂Si phase] and a Ti—Al base intermetallic compound is formedin the matrix of an [Al/Zn/Zn₂Mg ternary eutectic structure] in theplating layer.

The [Al/Zn/Zn₂Mg ternary eutectic structure] herein is a ternaryeutectic structure of an Al phase, a Zn phase and an intermetalliccompound Zn₂Mg phase. The Al phase forming the ternary eutecticstructure corresponds, for example, to an [Al″ phase] (Al solid solutiondissolving a Zn phase, and containing a small amount of Mg) at hightemperature in an Al—Zn—Mg ternary equilibrium state diagram. The Al″phase at high temperature usually appears at room temperature as a fineAl phase and a fine Zn phase in separation. Moreover, the Zn phase inthe ternary eutectic structure dissolves a small amount of Al, andfurther dissolves in some cases a small amount of Mg (Zn solidsolution). The Zn₂Mg phase in the ternary eutectic structure is anintermetallic compound phase present near a Zn content of about 84% byweight in a Zn—Mg binary equilibrium state diagram. As long as the statediagram is observed, it is thought that Si and Ti form no solid solutionwith each phase, or extremely small amounts of Si and Ti form a solidsolution therewith even when a solid solution is formed. Because theamounts cannot be definitely distinguished by conventional analysis, theternary eutectic structure composed of the three phases is representedby an [Al/Zn/Zn₂Mg ternary eutectic structure].

Furthermore, the [Al phase] is a phase that appears to be an islandhaving a distinct boundary in the matrix of the above ternary eutecticstructure. The phase corresponds, for example, to an [Al″ phase] (Alsolid solution dissolving a Zn phase, and containing a small amount ofMg) at high temperature in the Al—Zn—Mg ternary equilibrium statediagram. The Al″ phase at high temperature dissolves Zn and Mg with theamounts differing and depending on the concentrations of Al and Mg inthe plating bath. The Al″ phase at high temperature usually separatesinto a fine Al phase and a fine Zn phase at room temperature. Anisland-like shape observed at room temperature may be taken as ruins ofthe Al″ phase at high temperature. As long as the state diagram isobserved, it is thought that Si and Ti do not form a solid solution withthe phase, or the amounts are extremely small even when they form asolid solution therewith. However, because conventional analysis cannotdefinitely determine the amounts, the phase derived from the Al″ phaseat high temperature and having the ruins of the shape of the Al″ phaseis termed an [Al phase] in the present invention. The [Al phase] can bedefinitely distinguished from the Al phase forming the above ternaryeutectic structure by microscopic observation.

Furthermore, the [Zn phase] is a phase that appears to be an islandhaving a distinct boundary in the matrix of the above ternary eutecticstructure. Actually, the [Zn phase] sometimes dissolves a small amountof Al and further dissolves a small amount of Mg. As far as the statediagram is observed, it is thought that Si and Ti form no solid solutionwith the phase, or that the amounts are extremely small even when theyform a solid solution therewith. In a microscopic observation, the [Znphase] can be definitely distinguished from the Zn phase forming theabove ternary eutectic structure.

Moreover, the [Zn₂Mg phase] is a phase that appears to be an islandhaving distinct boundary in the matrix of the above ternary eutecticstructure, and actually dissolves a small amount of Al, sometimes. Asfar as the state diagram is observed, it is thought that Si and Ti formno solid solution with the phase, or that the amounts of Si and Ti areextremely small even when they form a solid solution therewith. In amicroscopic observation, the [Zn₂Mg phase] can be clearly distinguishedfrom the Zn₂Mg phase forming the above ternary eutectic structure.

Furthermore, the [Mg₂Si phase] is a phase that appears to be an islandhaving a distinct boundary in the solidified structure of the platinglayer. As far as the state diagram is observed, it is thought that Zn,Al and Ti form no solid solution with the phase, or that even when theyform a solid solution therewith, the amounts are extremely small. In amicroscopic observation of the plating layer, the [Mg₂Si phase] can beclearly distinguished.

Furthermore, the Ti—Al base intermetallic compound is a phase thatappears to be an island having a distinct boundary in the solidifiedstructure of the plating layer. As far as the state diagram is observed,the intermetallic compound is thought to be TiAl₃. However, because Siis observed when the Ti—Al base intermetallic compound is analyzed inthe plating layer to which Si is added, it is thought that the Ti—Albase intermetallic compound in the plating layer is TiAl₃ that dissolvesSi or Ti(Al_(1-x)Si_(x))₃ (X=0 to 0.5) in which Si is substituted forpart of Al.

The Ti—Al base intermetallic compound in the hot-dip galvanized steelproduct of the present invention is characterized in that theintermetallic compound is present in the [Al phase], [Zn₂Mg phase] and[Zn phase]. The contained form of the Ti—Al base intermetallic compoundis restricted to sites in the [Al phase], [Zn₂Mg phase] and [Zn phase]because a Ti—Al base intermetallic compound present in sites other thanthe above sites cannot improve the surface smoothness and formability.The Ti—Al base intermetallic compound present in sites in the [Alphase], [Zn₂Mg phase] and [Zn phase] is thought to improve the surfacesmoothness and formability for the following reasons: the Ti—Al baseintermetallic compound becomes nuclei of the [Al phase], [Zn₂Mg phase]and [Zn phase], promotes crystallization of these crystals, and manyfine structures are formed. That is, when the crystals become fine, therecesses and protrusions of the plating layer surface are suppressed. Asa result, the surface becomes smooth, and the friction coefficient isdecreased during forming due to a decrease in the deformation resistanceof the plating layer. It is thought that the formability of the platedsteel product is thus improved.

The effect is significant particularly in the [Al phase]. The platinglayer surface is smoothed and the friction coefficient is lowered byadjusting the size of a dendrite of the [Al phase] to 500 μm or less.The size is desirably up to 400 μm, and more desirably up to 300 μm.

As a result of examining metal structures in many plating layers, theyhave observed intermetallic compounds each having a size of severalmicrometers in most of the metal structures. FIG. 1 shows one example ofthe intermetallic compounds present in an [Al phase]. FIG. 1(a) is aphotomicrograph (magnification: 1,000×) of a plating layer of a platedsteel product in the present invention. FIG. 1(b) is a view showing thedistribution state of each structure in the photomicrograph. It isunderstood from the view that each structure can be definitely specifiedwith the help of a photomicrograph of the plating layer of a platedsteel product in the present invention.

In FIG. 1(a), a Ti—Al base intermetallic compound is observed in a phasecorresponding to an [Al″ phase] at high temperature in the Al—Zn—Mgternary equilibrium state diagram. The Al″ phase at high temperatureusually appears at room temperature as a fine Al phase and a fine Znphase in separation by a eutectoid reaction taking place at 277° C. inthe Al—Zn binary equilibrium state diagram. When a hypo-eutectoidreaction takes place herein, the Al″ phase crystallized at hightemperature starts to precipitate a Zn phase from a ternary eutectictemperature in the Al—Zn—Mg ternary equilibrium state diagram, and theremaining Al″ phase forms a eutectoid structure of a fine Al phase and afine Zn phase at temperature corresponding to the eutectoid reaction inthe Al—Zn binary equilibrium state diagram.

FIG. 2(a) is a photomicrograph (magnification: 3,500×) that enlarges theAl″ phase in FIG. 1. FIG. 2(b) is a view showing the distribution stateof each structure in the photomicrograph. It can be concluded from theobservation in detail of the Al″ phase that eutectoid structures thatare condensed precipitated Zn phases are present outside the Al″ phaseand around the Ti—Al base intermetallic compound.

Although there is no specific restriction on the size of theintermetallic compound in the present invention, the size observed bythe present inventors is up to 10 μm. Moreover, there is no specificlimitation on the proportion of the intermetallic compound present inthe plating layer structure. However, it is desirable that theintermetallic compound be present in an amount of at least 10% in one ofthe [Al phase], [Zn₂Mg phase] and [Zn phase].

There is no specific restriction on the method of adding theintermetallic compound. A method of dispersing fine powder of theintermetallic compound in the bath, a method of dissolving theintermetallic compound in the bath, and the like method, can be applied.When the plated steel product is produced in a continuous line in whichhot-dip galvanizing with a nonoxidizing furnace system is used, a methodof dissolving Ti in the plating bath is suitable. As the method ofdissolving Ti in a plating bath, a method of adding a Ti—Zn baseintermetallic compound is efficient because Ti can be dissolved at lowtemperature in a short period of time. Examples of the Ti—Zn baseintermetallic compound to be added include Zn₁₅Ti, Zn₁₀Ti, Zn₅Ti, Zn₃Ti,Zn₂Ti and ZnTi. When such an intermetallic compound is added to theplating bath singly or in a mixture of the intermetallic compound andZn, or an alloy of Zn—Al or Zn—Al—Mg, dissolved Ti crystallizes in theplating layer as a Ti—Al base intermetallic compound, and the compoundimproves the surface smoothness and formability.

Examples of the substrate steel product of the present invention includenot only steel sheets but also various steel products such as wire rods,shape steels, bar steels and steel tubes. Usable steel sheets includeboth hot rolled steel sheets and cold rolled steel sheets. Various steeltypes such as Al-killed steels, extra low carbon steels containing Ti,Nb, etc., high-strength steels containing strengthening elements such asP, Si and Mn and stainless steels can be used.

There is no specific restriction on the production process of the steelproducts in the invention, and various processes such as continuousplating of steel sheets, and hot-dip galvanizing of steel products andwire rods can be applied. When steel products are to be pre-plated withNi as a substrate layer, a conventional pre-plating process may beapplied. Because the plated products obtained in the present inventioneach have a plating layer excellent in surface smoothness even when thecooling rate is small, the process shows a significant effect in hot-dipgalvanizing in which the plated products are hardly cooled at a largecooling rate, and hot-dip galvanizing of thick steel products.

Although there is no specific limitation on the adhesion amount of theplating layer, the amount is desirably at least 10 g/m² in view of thecorrosion resistance, and up to 350 g/m² in view of the workability.

The zinc plating layer may also contain Fe, Sb, Pb and Sn singly or in amixture in an amount of 0.5% by mass or less in addition to the aboveelements. Moreover, even when the plating layer contains Ca, Be, Cu, Ni,Co, Cr, Mn, P, B, Nb, Bi and group III elements in a total amount of0.5% by mass or less, the effect of the invention is not impaired, andthe corrosion resistance of the plated steel products is sometimesfurther preferably improved, depending on the amount.

EXAMPLES Example 1

First, cold rolled steel sheets 0.85 mm thick were prepared. Each steelsheet was hot-dip galvanized for 3 sec in a plating bath at temperatureof 400 to 600° C. Amounts of addition elements in the bath were varied.The adhesion amount of a plating layer on one side of each steel sheetwas adjusted to 140 g/m² by N₂ wiping, and the plated steel sheet wascooled at a rate of 10° C./sec or less. Table 1 shows the platingcompositions of the plated steel sheets thus obtained. The cross sectionof each steel sheet was observed with a SEM, and Table 1 also shows theresults of observing the metal structure of the plating layer.

Each plated steel sheet was inclined at an angle of 10° to thehorizontal plane, and surface ground. Ti—Al base intermetallic compoundspresent in an [Al phase], a [Zn₂Mg phase] and a [Zn phase] weresubsequently observed with an EPMA.

The size of dendrites in [Al phases] in the plating layer was determinedby the following procedure. The surface of each plated steel sheet wasmapped by CMA, and major axes of dendrites in the Al map thus obtainedwere measured in an area of 5×5 cm. The major axes of five dendriteswere measured in order of decreasing magnitude. The average was used asthe size of dendrites in the [Al phases].

As to the smoothness, R_(a) and W_(CA) were measured with a surfaceroughness shape measurement apparatus (manufactured by Tokyo SeimitsuCo., Ltd.). The surface roughness of each steel sheet (only solidifiedby cooling) was measured at optional 3 sites under the conditionsexplained below, and the average was used.

Measurement probe: stylus tip having a curvature of 5 μm R

Measurement length: 25 mm

Cut off: R_(a) 0.8 mm, W_(CA) 0.8 to 8 mm

Driving speed: 0.3 mm/sec

Filter: 2 CR filter

The smoothness of each steel sheet was judged from the following scores.A steel sheet having a score of 4 was accepted.

4: R_(a) up to 1 μm, W_(CA) UP to 1 μm

3: R_(a) exceeding 1 μm, W_(CA) UP to 1 μm

2: R_(a) up to 1 μm, W_(CA) exceeding 1 μm

1: R_(a) exceeding 1 μm, W_(CA) exceeding 1 μm

The formability of each steel sheet was evaluated by a draw bead test. Adrawing load obtained under the following measurement conditions wasused, and the apparent friction coefficient was calculated.

Bead mold: round shape of a projected portion R 4 mm R, shoulder R 2 mmR

Sample size: 30 mm×300 mm

Slide length: 110 mm

Drawing speed: 200 mm/min

Pressing load: 600, 800, 1,000 kgf

The smoothness of each steel sheet was judged from the following scores.A steel sheet having a score of 3 was accepted.

3: less than 0.20

2: at least 0.20 to less than 0.21

1: at least 0.21

Each steel sheet was sprayed with 5% salt water at 35° C. for 1,000hours. When rust was not formed, the steel sheet was accepted. When rustwas formed, the steel sheet was rejected.

Table 1 shows the evaluation results. Because Sample No. 14 contained noTi—Al base intermetallic compound, an Al phase grew, and it was rejectedbecause of the smoothness and formability. Because Sample No. 15 had anexcessive Ti content, a Ti—Al base intermetallic compound condensed inthe surface, and it was rejected because of the smoothness andformability. Because Sample No. 16 had contents of Mg, Al, Si and Tioutside the scope of the present invention, it was rejected because ofthe corrosion resistance. Samples other than the above ones each showedgood smoothness, formability and corrosion resistance. TABLE 1Composition of hot-dip Metal structure Sample Zn coating layer (mass %)Mg₂Si Ternary Al Zn MgZn₂ Ti—Al base No. Mg Al Si Ti phase eutecticphase phase phase intermetallic cpd. 1 4 8 0.15 0.009 ◯ ◯ ◯ ◯ ◯ ◯ 2 5 100.2 0.009 ◯ ◯ ◯ ◯ ◯ 3 5 15 0.45 0.009 ◯ ◯ ◯ ◯ ◯ 4 3 6 0.05 0.009 ◯ ◯ ◯ ◯◯ 5 1 19 0.5 0.009 ◯ ◯ ◯ ◯ ◯ 6 1 4 0.005 0.009 ◯ ◯ ◯ ◯ ◯ 7 3 11 0.20.009 ◯ ◯ ◯ ◯ ◯ 8 3 11 0.2 0.001 ◯ ◯ ◯ ◯ ◯ 9 3 11 0.2 0.0002 ◯ ◯ ◯ ◯ ◯10 3 11 0.2 0.00005 ◯ ◯ ◯ ◯ ◯ 11 3 11 0.2 0.000001 ◯ ◯ ◯ ◯ ◯ 12 3 110.0002 0.009 ◯ ◯ ◯ ◯ ◯ 13 3 11 0.000001 0.009 ◯ ◯ ◯ ◯ ◯ 14 3 11 0.2 0 ◯◯ ◯ ◯ 15 3 11 0.2 0.12 ◯ ◯ ◯ ◯ ◯ 16 0 0.2 0 0 ◯ 17 3 6 0 0.01 ◯ ◯ ◯ ◯ ◯Sample No. Size of Al phase Smoothness Formability Corrosion resistanceNote 1 150 μm 4 3 Accepted Ex. 2 150 μm 4 3 Accepted Ex. 3 150 μm 4 3Accepted Ex. 4 100 μm 4 3 Accepted Ex. 5 200 μm 4 3 Accepted Ex. 6 100μm 4 3 Accepted Ex. 7 150 μm 4 3 Accepted Ex. 8 200 μm 4 3 Accepted Ex.9 300 μm 4 3 Accepted Ex. 10 350 μm 4 3 Accepted Ex. 11 450 μm 4 3Accepted Ex. 12 150 μm 4 3 Accepted Ex. 13 150 μm 4 3 Accepted Ex. 141200 μm  2 2 Accepted Comp. Ex. 15 200 μm 3 2 Accepted Comp. Ex. 16 — 43 Rejected Comp. Ex. 17 150 μm 4 3 Accepted Ex.

Example 2

First, cold rolled steel sheets 0.85 mm thick were prepared. Each steelsheet was hot-dip galvanized for 3 sec in a plating bath at atemperature of 520° C. Amounts of addition elements in the bath werevaried. The adhesion amount of a plating layer on one side of each steelsheet was adjusted to 140 g/m² by N₂ wiping, and the plated steel sheetwas cooled at a rate of 10° C./sec or less. Table 2 shows the platingcompositions of the plated steel sheets thus obtained. The cross sectionof each steel sheet was observed with a SEM, and Table 2 also shows theresults of observing the metal structure of the plating layer.

Each plated steel sheet was inclined at an angle of 10° to thehorizontal plane, and surface ground. Ti—Al base intermetallic compoundspresent in an [Al phase], a [Zn₂Mg phase] and a [Zn phase] weresubsequently observed with an EPMA. Moreover, as to Ti—Al baseintermetallic compounds present in an [Al phase], the presence orabsence of the Ti—Al base intermetallic compounds in a Zn—Al eutectoidstructure in which a precipitated [Zn phase] was condensed was observedwith an EPMA. Furthermore, whether a Ti—Al base intermetallic compoundcontained Si or not was observed by observing a Ti—Al base intermetalliccompound with an EPMA.

The adhesion of the plating layer was evaluated by the followingprocedure. An adhesive cellophane tape was affixed to each hot-dipgalvanized steel sheet subsequent to a Du Pont impact test, and the tapewas peeled off the steel sheet. The adhesion was evaluated by thefollowing criteria:

◯: the plating layer is not exfoliated;

Δ: the plating layer is exfoliated in an area of less than 10%; and

X: the plating layer is exfoliated in an area of at least 10%. In the DuPont test, an impact mold having a roundness of ½ inch at the tip wasused, and the test was carried out by dropping a 1-kg weight from aheight of 1 m.

The corrosion resistance of each plated steel sheet subsequent toworking was evaluated by the following procedure. A sample was subjectedto 1 T bending (bending a sample to be tested at an angle of 180° whilethe sample was being held). The bent portion of the sample was subjectedto SST for 1,000 hours. The rust formation state on the bent portion wasjudged according to the following scores. A steel sheet having a scoreof at least 3 was accepted.

5: rust formation area of less than 5%;

4: rust formation area of from at least 5% to less than 10%;

3: rust formation area of from at least 10% to less than 20%;

2: rust formation area of from at least 20% to less than 30%: and

1: rust formation area of at least 30%.

Table 2 shows the evaluation results. Because the addition amounts of Aland Si in Sample No. 2 were outside the scope of the present invention,Sample No. 2 was rejected because of the adhesion. The other sampleseach showed good adhesion of the plating layer and good corrosionresistance after working. Plated steel sheets that had a plating layercontaining Si showed particularly good adhesion and corrosion resistanceafter working. TABLE 2 Composition of hot-dip Metal structure Sample Znplating layer (mass %) Mg₂Si Ternary Al Zn MgZn₂ Ti—Al base No. Mg Al SiTi Phase eutectic phase phase phase intermetallic cpd. 1 3 12 0.2 0.009◯ ◯ ◯ ◯ Ti(Al_(0.85)Si_(0.15))₃ 2 3 12 0 0.009 ◯ ◯ ◯ TiAl₃ 3 3 6 0 0.01◯ ◯ ◯ TiAl₃ 4 3 6 0.002 0.01 ◯ ◯ ◯ ◯ Ti(Al_(0.85)Si_(0.15))₃ Sample Sitewhere Ti—Al base intermetallic Corrosion resistance No. compound in Alphase was present Adhesion after working Note 1 In Zn—Al eutectoidstructure where Zn ◯ 5 Ex. phase was condensed 2 In Zn—Al eutectoidstructure where Zn X 3 Comp. Ex. phase was condensed 3 In Zn—Aleutectoid structure where Zn Δ 4 Ex. phase was condensed 4 In Zn—Aleutectoid structure where Zn ◯ 5 Ex. phase was condensed

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, highlycorrosion-resistant plated steel products that show excellent surfacesmoothness and formability, without forming protrusions and recesses inthe surfaces even when the solidification rate of the plating layers wassmall, can be produced.

1. A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, having on the steelproduct surface a zinc alloy plating layer composed of 4 to 10% by massof Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti and the balanceof Zn and unavoidable impurities, the plating layer having a metalstructure in which one or at least two of the [Al phase], [Zn₂Mg phase]and [Zn phase] are present in a mixture in the matrix of an [Al/Zn/Zn₂Mgternary eutectic structure], and the plating layer containing a Ti—Albase intermetallic compound in one or at least two of the [Al phase],[Zn₂Mg phase] and [Zn phase].
 2. A highly corrosion-resistant hot-dipgalvanized steel product excellent in surface smoothness andformability, having on the steel product surface a zinc alloy platinglayer composed of 4 to 22% by mass of Al, 1 to 5% by mass of Mg, up to0.1% by mass of Ti, up to 0.5% by mass of Si and the balance of Zn andunavoidable impurities, the plating layer of the plated steel producthaving a metal structure in which an [Mg₂Si phase], an [Al phase] and a[Zn₂Mg phase] are present in a mixture in the matrix of an [Al/Zn/Zn₂Mgternary eutectic structure], and the plating layer containing a Ti—Albase intermetallic compound in one or at least two of the [Al phase] and[Zn₂Mg phase].
 3. A highly corrosion-resistant hot-dip galvanized steelproduct excellent in surface smoothness and formability, having on thesteel product surface a zinc alloy plating layer composed of 4 to 22% bymass of Al, 1 to 5% by mass of Mg, up to 0.1% by mass of Ti, up to 0.5%by mass of Si and the balance of Zn and unavoidable impurities, theplating layer of the plated steel product having a metal structure inwhich an [Mg₂Si phase], an [Al phase], a [Zn₂Mg phase] and a [Zn phase]are present in a mixture in the matrix of an [Al/Zn/Zn₂Mg ternaryeutectic structure], and the plating layer containing a Ti—Al baseintermetallic compound in one or at least two of the [Al phase], [Zn₂Mgphase] and [Zn phase].
 4. A highly corrosion-resistant hot-dipgalvanized steel product excellent in surface smoothness andformability, having on the steel product surface a zinc alloy platinglayer composed of 4 to 22% by mass of Al, 1 to 5% by mass of Mg, up to0.1% by mass of Ti, up to 0.5% by mass of Si and the balance of Zn andunavoidable impurities, the plating layer of the plated steel producthaving a metal structure in which an [Mg₂Si phase], an [Al phase] and a[Zn phase] are present in a mixture in the matrix of an [Al/Zn/Zn₂Mgternary eutectic structure], and the plating layer containing a Ti—Albase intermetallic compound in one or two of the [Al phase] and [Znphase].
 5. A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, wherein the Ti—Al baseintermetallic compound according to any one of claims 1 to 4 is TiAl₃.6. A highly corrosion-resistant hot-dip galvanized steel productexcellent in surface smoothness and formability, wherein the Ti—Al baseintermetallic compound according to any one of claims 1 to 4 isTi(Al_(1-x)Si_(x))₃ (wherein X=0 to 0.5).
 7. The highlycorrosion-resistant hot-dip galvanized steel product excellent insurface smoothness and formability according to any one of claims 1 to4, wherein the Ti—Al base intermetallic compound contained in an [Alphase] in the plating layer is present in a Zn—Al eutectoid reactionstructure in which Zn phases are condensed.
 8. The highlycorrosion-resistant hot-dip galvanized steel product excellent insurface smoothness and formability according to any one of claims 1 to4, wherein the size of a dendrite in an [Al phase] in the plating layeris up to 500 μm.
 9. A process for producing the highlycorrosion-resistant hot-dip galvanized steel product excellent insurface smoothness and formability according to any one of claims 1 to4, comprising the step of adding a Ti—Zn base intermetallic compound toa plating bath.